Comparing with a projection cathode-ray tube display device, a projection type liquid crystal display device has more attractive features such as a wider range of color reproduction, better mobility due to small size and light weight, no need for a convergence adjustment as it is not affected by earth magnetism, and so on. Since the screen size of the projection type liquid crystal display device can be increased easily, the projection type liquid crystal display device is expected to prevail as a mainstream of home-use image display device in the future.
Among color projection type image display modes utilizing liquid crystal display element, there is three-panel mode in which three panels of liquid crystal display element are respectively used for three primary colors, and a single-panel mode in which only one panel of liquid crystal display element is used. In the three-panel mode, an optical system and three panels of liquid crystal display element are independently provided, wherein the optical system divides a white beam into three primary colors, R, G and B, and the respective color beams are controlled by the three panels of liquid crystal display element, so as to form an image. Full color display is achieved by optically superimposing images of the respective colors.
Some of the advantages of the three-panel mode are that light being emitted from a white beam source is efficiently utilized, and that the color purity is high. However, it is difficult to reduce the cost and size of a liquid crystal display device of the three-panel mode, as it requires the color separation function and the image formation function, which consequently makes its optical system to become more complicated with a larger number of components.
On the other hand, a liquid crystal display device of the single-panel mode utilizes only one panel of liquid crystal display element with a color filter pattern shaped in mosaic, stripe, or the like for three primary colors. Images are displayed by projecting light on the liquid crystal display element, using an optical system for projection. The image display device of the single-panel mode is suitable for low-cost and small projection systems, because it requires only one panel of liquid crystal display element, and the structure of its optical system is simpler than one in the three-panel mode.
However, a disadvantage of the liquid crystal display device of the single-panel mode is that it can use only a ⅓ of incident light due to absorption or reflection of light that occurs at its color filter. For a solution, Japanese Unexamined Patent Application No. 181487/1995 (Tokukai 7-181487; published on Jul. 21, 1995) discloses a color-filterless single-panel mode liquid crystal display device in which two layers of micro-lens arrays are used.
A single-panel mode liquid crystal display device is described below with a reference to FIG. 1. In the liquid crystal display device of the single-panel mode shown in FIG. 1, dichroic mirrors 5G, 5R and 5B are arranged in a sector form, and first and the second micro-lens arrays 6 and 7 are arranged on the side of a light source in a liquid crystal display element 20. The dichroic mirrors divide white light from a white beam source 2 into R, G and B, and cause the divided beams to enter the first and second micro-lens arrays 6 and 7 at different angles. After passing the first micro-lens array 6, the light beams of the respective colors are refracted by the second micro-lens array 7 so that the principal rays of the respective colors R, G and B separated by the dichroic mirrors 5G, 5R, and 5B become substantially parallel to one another. The respective light beams refracted by the second micro-lens array 7 separately fall on liquid crystal regions driven by signal electrodes that are independently impressed with color signals corresponding with R, G and B.
The single-panel mode liquid crystal display device does not use an absorbing color filter; therefore, not only the efficiency of using light improves but also the principal rays of respective colors after passing the micro-lens arrays are made substantially parallel to one another. In other words, the single-panel mode liquid crystal display device provides remarkably bright images by restraining diffusion of the principal rays of the respective colors before they reach a projection lens 10, and by preventing decrease in light quantity caused by scattering at the projection lens 10.
In the double-layer micro-lens array described in the foregoing publication, micro-lens arrays are attached on both sides of a glass substrate.
Fabrication method of a double-layer micro-lens array is disclosed in Japanese Unexamined Patent Application No. 90336/1997 (Tokukai 9-90336; published on Apr. 4, 1997). In this method, first and second micro-lens arrays are fabricated separately, and the lens surface of the first micro-lens array is jointed with the polished surface of the second micro-lens array.
In the prior art thus described, a double-layer micro-lens array is produced by forming micro-lens arrays on both sides of a glass substrate, or by jointing two micro-lens arrays together. This, however, poses the problem of high manufacturing cost associated with difficulties in optical-axes alignment, as described below.
The micro-lens array substrate, which possesses two layers of micro-lens array, requires a process of optical axes alignment for two of the micro-lens arrays. In other words, the two micro-lens arrays must have the same angle in all directions, including vertical, horizontal and rotational directions, in order to ensure optical properties of the lenses.
However, the required accuracy for the optical-axes alignment is ±1 μm, due to a microscopic structure of the lens pattern. This has made the production of double-layer micro-lens array substrates extremely difficult.
Moreover, a middle layer between the two layers of micro-lens array is another cause of difficulty in the optical-axes alignment. Specifically, the presence of a gap between the two layers of lens pattern prevents the alignment marks for position adjustment of the two layers to be simultaneously focused for inspection. The optical axes alignment can be carried out by providing separate alignment mark observation systems for the respective layers. However, in this case, the alignment mark observation systems still require accurate optical-axes alignment. Thus, as described above, a middle layer between the two layers of micro-lens array has also been a cause of high cost of a positioning apparatus.
The present invention was made in view of the foregoing problems and an object of the invention is to provide a micro-lens array substrate, which can be produced by a simple process with easy optical-axes alignment, and to provide a production method of a micro-lens array substrate, as well as a projection type liquid crystal display device.